Introduce cc_baremetal_defaults

Add a way to easily share configuration across all C/C++ libraries
targeting a "baremetal" environment (typically AArch64 EL1) but
unfortunately re-using the cc infrastructure, which actually targets
Android EL0. This will be used to restrict the compiler flags in a
follow-up.

Note: No functional change intended.

Test: m
Change-Id: I1160d87088090d2f429f33ea53385329e5caa2ce